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The nearly 9 percent reduction in annual carbon emissions in the US between 2005 and 2011 is unlikely to continue without major changes in how energy is currently produced and used, according to a study by Climate Central, which conducts scientific research on climate change.

The “Can US Carbon Emissions Keep Falling?” study said the economic recession was the primary reason annual US carbon emissions from fossil fuels dropped more than 500 million metric tons from 2005 to 2011. Once the economy rebounds, the decline in emissions will likely be overtaken by a growing population and rising incomes, said Eric Larson, the author of the study.

Other drivers for the decline in emissions included an increase in natural gas production and the related decline in coal-fired electricity generation, energy efficiency and growth in renewables, particularly wind power, the Climate Center said. However, those factors could either change or are not significant enough to continue the rate of decline in emissions, the study said.

Low natural gas prices have caused a recent reduction in coal-generated electricity and left the existing US fleet of coal-fired power plants operating at only about 50 percent capacity. However, the study forecasts gas prices will not remain at historic lows and eventual price increases will make firing up idle coal capacity more competitive, leading to increased CO2 emissions.

The study found doubling new car MPG by 2025 will produce about a 40 percent cut in overall fleet gasoline consumption, but not further reductions since new cars are a small fraction of the total fleet. The 40-percent reduction assumes the total number of vehicle miles driven by a growing population will be no higher than today, which is unlikely, the study said.

Climate Central, a non-advocacy organization of scientists and journalists, launched its web site during the 15th United Nations Climate Change Conference (UNFCCC) in Copenhagen in 2009 to help explain the science behind global warming. This summer, Climate Central released an interactive map summarizing the average temperatures of each of the 48 contiguous United States for the last 100 years.